Resource Predictability and Movement Strategies in Ungulates: Does Temporal Uncertainty Lead to Nomadism?
The movements of all animals are affected by their need for resources such as food. Where and how quickly animals move often depends on where the best vegetation resources can be found and how predictable this food is from year to year. Some ungulate species with predictable environments migrate seasonally; caribou and wildebeest are examples of this. A few other species - gazelles, for example - appear to make large-scale, long-range movements that are seemingly unpredictable. This 'nomadism' likely occurs when the availability and location of resources varies considerably by season and by year. To make sense of these seasonal and annual movement strategies, researchers will combine: 1) theoretical computer models, 2) landscape-scale satellite images of vegetation, and 3) detailed movement data from individual gazelles in Mongolia (new field studies) and caribou in Alaska (historical data). By understanding long-distance animal movements, we will be in a much better position to know how environmental variability affects animal behavior. The conservation of these species and their habitats depends on understanding their large-scale movement patterns and ecology. Our research will help identify the mechanisms by which animals 'read' their environment to know when and where to move over complex landscapes.
Funding from this grant allowed researchers to study the quasi-migratory movements of gazelles in Mongolia: Long distance animal movements in grasslands ecosystems, such as migrations of bison across the prairie, of wildebeest in the Serengeti have long intrigued naturalists, researchers and the general public alike. However many historic migrations have now ceased to exist because formerly vast ranges have been broken up into small parcels by human development. One expansive grassland comprises the steppes of Eastern Mongolia. In these steppes, about which comparatively little is known from a biological perspective, the most abundant species of large herbivore is the Mongolian gazelle (Procapra gutturosa), of which there are approximately 1 million individuals in the wild. In this study we tracked 36 gazelles over several years and found a unique movement behavior that differs from other large herbivores. In particular, we discovered that gazelles move long distances but that their movements don’t follow regular ‘migratory’ patterns between distinct summer and winter ranges (like wildebeest, for example). Instead, individuals appear to move year-round in irregular and seemingly chaotic ways, without repeating movements of previous years and independent of other members of their species. These movements, which we label ‘nomadism’, are probably driven by highly variable rainfall events leading to transient green up of steppe vegetation. Such highly dynamic resource distributions translate into huge area requirements for gazelle populations. We found that a single individual alone can roam over an area several times the size of Yellowstone National Park. We also developed a series of computational and mathematical models that explored theoretical aspects of animal navigation and migration. These touched on such as issues as group leadership, the environmental triggers favoring range residency versus migration, and orientation behavior in response to stimuli from the environment. Landscapes where the spatial distribution of resources exhibits a pattern that repeats predictably over time were found to favor animals that use spatial memory as a primary navigation mechanism. When resources changed in a non-predictable fashion, movement behaviors incorporating random searches were more common. In recent years, efforts have begun to bring economic development to the steppes of Eastern Mongolia via oil and mineral extraction and its corresponding transportation infrastructure such as road and railroad construction. Such development will hinder movements of the wandering gazelles, which are particularly sensitive to any sort of barriers on their landscapes. The continuity of vast unfragmented ranges is critical for the survival of this species and their unique broad-ranging movement behavior demonstrated by this research. We found that gazelle area requirements are simply too large to effectively conserve them within single protected areas. Instead, developing landscape-level strategies pertaining to the entire steppe will be key to effectively conserving Mongolian gazelles for the future.
